Top N queries and disbursion

We have finished our work on optimization of Top N queries (with clauses
STOP AFTER ... FOR EACH ... RANK BY ...) now it's time of validating it
with performance tests: since we are working on snapshots of the 6.6
release (now we are using snapshot dated 9/13/99) we are afraid of
instability problems to affect the results. Could you give us any
suggestion about this? We are quite close to the degree day, so we have
to optimize time usage... 
BTW, first results seem to be interesting.

We would ask you for a last thing.
We need to estimate the number of distinct values of an attribute. We
thought 1/disbursion was the right solution, but the results were quite
wrong:
with 100 distinct values of an attribute uniformly distribuited in a
relation of 10000 tuples, disbursion was estimated as 0.002275, giving
us 440 distinct values. We have seen this disbursion estimates
result also in bad join selectivities estimate.
Could this be due to a bad disbursion estimate or is our solution
completely wrong?
Thanks a lot

Roberto Cornacchia
Andrea Ghidini